Char is used as a fuel and is manufactured by partially pyrolyzing coal or other carbonaceous materials. Char contains sulfur which is objectionable because upon combustion, the sulfur forms sulfur dioxide, an air pollutant. Governmental air purity standards limit the use of char of fuels containing relatively low concentrations of sulfur. The presence of sulfur concentrations which exceed such limits has restricted the use of char as a fuel. The need for a relatively sulfur-free char has therefore become very important, especially in view of dwingling supplies of oil and natural gas and abundant supplies of coal.
The sulfur content of char is present in many forms such as organic sulfur, pyritic sulfur and sulfide sulfur. Organic sulfur is the sulfur which forms a part of organic molecules contained in the char. Pyritic sulfur is the sulfur that forms a part of iron pyrite, FeS.sub.2, found in char. Sulfide sulfur is the sulfur that forms a part of inorganic sulfur compounds found in char, such as, for example, FeS and CaS. Pyritic sulfur is an inorganic sulfur but is generally not comprehended by the term "sulfide sulfur" as used in the art.
Sulfur bound to organic molecules is probably the most difficult form of sulfur to completely remove from char due to the strong, complex organic bonds which tightly hold onto it and stubbornly refuse to give it up. Methods are being devised to significantly reduce the content of pyritic and sulfide sulfur in char, and while such methods may also cause some reduction in the organic sulfur content they have not been able to substantially reduce the organic sulfur in char.